Zinc alloy die-casting is widely used in various decorative aspects, such as furniture accessories, building decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc., so the surface quality of the casting is higher, and good surface treatment performance is also required.

Defect characterization: there are bubbles on the surface of die-casting parts, which are found after die-casting, exposed after polishing or processing, and appear after oil spraying or electroplating. Cause:

1. Caused by holes: mainly pores and contraction mechanism, pores are often round, and contraction is mostly irregular shape.

(1) Causes of pores: In the process of filling and solidification of metal liquid A, holes are produced on the surface or inside of the casting due to gas intrusion. B Intrusion of volatile gas from paint. C alloy liquid gas content is too high, solidification precipitation.

When the gas in the cavity, the gas volatilized by the coating and the gas precipitated by the alloy solidification, the gas in the mold exhaust is not good, and finally stays in the pores formed in the casting.

(2) Causes of shrinkage cavity: in the solidification process of liquid metal a, shrinkage cavity is generated due to volume reduction or the final solidification site can not be fed by liquid metal. B. Uneven thickness of castings or local overheating of castings causes slow solidification of a certain part and concave surface when volume shrinks. Due to the existence of porosity and shrinkage cavity, the hole may enter the water when the die casting surface treatment, when the painting and electroplating after baking, the gas in the hole heated expansion; Or the water in the hole will steam, volume expansion, resulting in blisters on the surface of the casting.

2. Caused by intergranular corrosion:

The harmful impurities in zinc alloy composition: lead, cadmium and tin will gather at the grain boundary leading to intergranular corrosion, metal matrix is broken due to intergranular corrosion, and electroplating accelerates this scourge, the parts affected by intergranular corrosion will expand and the coating will be jacking, resulting in blistering on the casting surface. Especially in humid environment, intergranular corrosion will cause casting deformation, cracking and even broken.

3. Crack cause: water grain, cold grain, hot crack.

Water grain and cold grain: in the filling process of metal liquid, the metal liquid that first enters the mold wall solidifies prematurely, and the metal liquid that enters the mold cannot be fused with the solidified metal layer as a whole, resulting in overlapping grain and stripe defects on the surface junction of the casting. Water grain is generally in the shallow surface of the casting; The cold insulation may penetrate into the inside of the casting.

Hot crack: a when the casting thickness is uneven, the solidification process produces stress; B premature ejection, insufficient metal strength; C. Uneven force during eaving; d. Coarse-grained grains due to high die temperature; E Harmful impurities are present.

All of the above factors are likely to produce cracks.

When there are water lines, cold lines and hot cracks in die casting parts, the solution will penetrate into the cracks during electroplating, and it will be converted into vapor when baking, and the electric coating will be bubbled up by air pressure.

Defect solution:

Control air hole, the key is to reduce the amount of gas in mixed with the casting, the ideal metal flow should be accelerated by the nozzle after cone and sprue into the mold, forming a smooth and metal flow in the same direction, the taper runner design, namely water flow should be accelerated to from the nozzle to the gate to reduce gradually, can achieve this goal. In the filling system, mixed with liquid mixed gases is due to turbulence and metal and form pores, from liquid metal into the cavity by the casting system in the study of simulation of die casting process, obvious sprue and increasing sharp change in the cross-section of runner, will appear the metal flow turbulence and gas volume, steady liquid metal to favour gas from the runner and the cavity into the overflow tank and exhaust slot, discharge mode.

For shrinkage cavity: all parts of the die casting solidification process as far as possible at the same time uniform heat dissipation, solidification. Shrink cavity can be avoided by reasonable nozzle design, inner gate thickness and location, mold design, mold temperature control and cooling. For intergranular corrosion phenomenon: mainly to control the content of harmful impurities in alloy raw materials, especially lead<0.003%. Pay attention to the impurities in the waste.

For water lines and cold lines, the mold temperature can be increased, the speed of the inner gate can be increased, or the overflow groove can be increased in the cold lines to reduce the appearance of cold lines.

For hot crack: die casting thickness should not change sharply to reduce stress generation; The relevant die casting process parameters are adjusted; Reduce mold temperature.